Percy Weasley and Cornelius Fudge from Harry Potter both start out fairly nice, if a bit pompous then turn against Dumbledore. Of the two, Percy gets better. Fudge seemingly also returns to normal as of the 6th book when he loses his position as Minister Of Magic.

Ron temporarily went through this in Deathly Hallows, where he leaves Harry and Hermione on their own...only to come back and apologize. He makes up for this by destroying the Horcrux that caused him to act like a jerk.

Even Harry himself isn't immune. His already-short temper gets a LOT worse in Order of the Phoenix due to both his age and being shunned by almost everyone he knows thanks to the Ministry slandering him.

Adam Parrish, who was easily one of the nicest characters in The Raven Boys, had this happen in The Dream Thieves. He gets better by the end of the book and it's implied to be a result of his sacrifice to Cabeswater and his father's abuse taking its toll on him but the contrast is still jarring. Heck, for most of the book he makes Ronan look like a sweetheart in comparison.

Nick Carraway from The Great Gatsby becomes more and more sarcastic toward the end of the novel, since the girl people tried to hook him up with turned out to be a fraud and his friend Gatsby got killed by his cousin's abusive husband's lover's widower. Kind of a lot to swallow.

Nathaniel in The Bartimaeus Trilogy starts out as a kid you can sympathize with and becomes a complete Jerkass as he becomes more and more invested in the magicians' society.

The Reynard Cycle: Reynard is portrayed far less sympathetically as the series goes on, committing acts that rankle even his best friend. This is one of the main features of the series, seeing as it is a Deconstruction of the Loveable Rogue trope.

Rand al'Thor in The Wheel of Time gets harder and colder as the series goes on, a combination of the trials of his position and his traumatic experiences (he's pretty mad as of book 7 and spirals downward into insanity from then on). This gets really bad in The Gathering Storm, where he starts alienating even his closest friends and allies to the point of nearly killing his own father out of misplaced rage and paranoia, before finally having a breakthrough while having a good old therapeutic Straw Nihilist rant and conversation with the voice in his head upon the site of the death of his last incarnation 3000 years ago..

Onestar from Warrior Cats. He's a Nice Guy early on, but after becoming the leader of WindClan, he becomes more of a jerk as he attempts to prove his Clan's independence.

Clear Sky went through this after creating his community. In fact, he gets mad at Gray Wing for accidentally killing Fox...though that was his own fault, for HE was the one who told Fox to attack him. Another sign that shows this is when he kicks Jagged Peak out for breaking his leg and refusing to take his son Thunder into the community.

Tallstar after his father Sandgorse dies. He's so upset and angry about it that he takes it out on the rogues...especially Sparrow, whom he blames for Sandgorse's death. Fortunately, Sparrow tells him the entire truth, and Talltail (which he was called back then) apologizes.

Sisterhood Series by Fern Michaels: Oh, man! Many of the characters start out as relatively nice, and then become more and more like Jerkasses as the series goes on. Charles Martin notes in the book Vanishing Act that the Vigilantes are treating him with little respect, when they used to defer to his judgement before. Charles is hardly a saint himself in terms of behaviour. However, their attitudes come back to bite them hard in the book Deja Vu, and they seem to have dropped the Jerkassitude (ha, ha!) by the book Home Free.

King Orrin in the Inheritance Cycle. He begins as a somewhat dissolute, but brave and intrepid young man and a vital component of the Varden (Orik even says that the Varden couldn't exist without his hard and dangerous work). However, stress, as well as feeling under-appreciated and overshadowed lead to a drinking problem in between Brisingr and Inheritance, which leads to his taking a level in jerkass.

Argubly SamTemple. In the first book he's described as the third most kind person in a town of 400 kids (Mary and Edilio just beating him). He, even though he knew the risks, took on the task of looking after everyone in the FAYZ and was sympathetic and sincere to all the kids, and didn't seem to have any alterior motives of all but to make sure everyones safe. he didn't want to be the leader per se, but he wanted to help people through their problems and didn't ask for anything in return, even though the kids were usually complete asswipes to him. Fastforward to FEAR and he's (in no particular order); cheated on his girlfriend and then tried to blame it on her, made out with a girl nearly 4 years younger than him, tried to pressure his girlfriend into non-consensual sex, refused to save the town unless the dying folk adknowledged that they "needed him", told his best friend that he didn't care about anyone and got drunk on the illegal booze HE outlawed. Justified in that, 9/10 of the people there are complete douches too, and after spending so long in such a terrible environment, you can't really blame him if it rubbed off on him too.

Another debatable one is Astrid Ellison. She started the series with very strong morals which she loyally stuck to, and often was the one to break up fights and encourage equality between the freaks and the normals. Sure, she had a few jerkass moments which might or might not be justified, but she certainly wasn't a shit head to half the magnitude or consistancy as 90% of Perdido Beach, and may even of been a rolemodel to real life girls. Then she got seriously, seriously bitchy. It was pretty bad in LIES, when she humiliated Mary by telling the entire town about her bulimia just to distract from the fact she lied to the entire town, but PLAGUE just took the trope up to 11. throwing her brother out of a window might be the most exaggerated example of this trope. Now; did she get better or worse during FEAR? That's the real question.

Brianna could qualify. In book 1 she was a sweet, somewhat hyper but nice girl. In book 5, she's a mouthy, egocentric borderline sociopath.

Could be argued that all characters (sans Diana) had to do this in order to survive.

Sarai Balitang of the Daughter of the Lioness duology. She stops being Nice to the Waiter in the second book, stops listening to anyone else's opinions and accuses them of being complacent and unwilling to act (unable to realize they're actually plotting revolution), and at one point threatens Aly with a riding crop. Easily justified by the fact that her father died horribly in the first book and she's not in a place where dealing with that properly can be done.

In Provost's Dog, Beka's mentor Tunstall goes from being the paternal and easygoing half of his and Goodwin's Odd Couple in Terrier to a grouch who goes out of his way to antagonize the group's Nice Guy mage at any opportunity and harshly reprimands Beka for getting emotional over the fact that they can't do anything for a giant heap of brutally murdered people. Probably due to the insecurities that come with him aging, career prospects, and the nasty comments his relationship with Lady Sabine attracts. Those are the reasons he turns traitor, after all.

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